Electrophotographic laser printing technology employs a toner containing pigment components and thermoplastic components for transferring a latent image formed on selected areas of the surface of an insulating, photoconducting material to an image receiver, such as plain paper, coated paper, transparent substrate (electrically conducting or insulative), or an intermediate transfer medium.
There is a demand in the laser printer industry for multi-colored images. Responding to this demand, designers have turned to liquid toners, with pigment components and thermoplastic components dispersed in a liquid carrier medium, usually special hydrocarbon liquids. With liquid toners, it has been discovered that the basic printing color (yellow, magenta, cyan, and black) may be applied sequentially to a photoconductor surface, and from there to a sheet of paper or intermediate transfer medium to produce a multi-colored image.
Direct transfer of images from toner on the photoconductor drum to the image receiver is well-known in the an of conventional dry electrostatic printing. In such an approach, electrostatic forces attract the toner to the drum from the toner source, and subsequent electrostatic forces on the image receiver are stronger than the adhesion of the toner to the drum, and thereby attract the toner to the image receiver, where it is subsequently fused.
However, under certain conditions, such as when the surface of the final image receiver (plastic, rubber, coated paper, plain paper, and the like) is too rough to provide enough contact between the toner layer and the image receiver surface or when the particle size of the toner is too fine to be efficiently transferred electrostatically, then the image transfer efficiency is reduced. To address these problems, intermediate transfer procedures have been developed in electrophotography. Such intermediate transfer procedures are used to improve the image transfer efficiency.
Poor transfer occurs when (1) the surface of the image receiver is too rough, as noted above, (2) the particle size is less than 3 .mu.m, such as the case of liquid developer, and (3) a film-forming liquid toner is used, especially when a dried image is made of the liquid toner (a film-forming toner is a toner having a high polymeric binder content).
In indirect transfer, the toner image is transferred by an electric field or by a thermally-assisted pressure transfer (not requiting a transfer bias) into an intermediate surface and then from the intermediate surface into the final image receiver (plain paper, plastic transparency, coated paper, and the like). Thus, the transfer promotion driving forces can be (1) an electric field, (2) a heat source, (3) a pressure source, and (4) a pressure aid (for example, a stickier image receiver will pick up the toner better).
The advantage of indirect transfer is a significant improvement of transfer efficiency when the transfer condition is not well-established, under the conditions outlined above. However, the disadvantages of indirect transfer are that (1) it adds another step, thereby complicating the imaging process, which can reduce the reliability of the imaging process, since the step involving the intermediate materials requires additional considerations as to process, life, and maintenance of the intermediate materials, (2) it is more expensive, and (3) that the transfer efficiency can be reduced with increased transfer steps.
To avoid the disadvantages of indirect transfer and to deal with the problems of smaller particle sizes of the toner, non-electrostatic transfer methods have been developed. For example, in thermally-assisted transfer (with possibly electrostatically-assisted transfer), the image receiver is heated, typically to a temperature within the range of about 60.degree. to 90.degree. C. and is pressed against the toner particles. The toner particles are fused to each other at the point of contact, but are not melted. Examples of this process are disclosed in, e.g., U.S. Pat. No. 4,927,727, issued to Rimai et at, U.S. Pat. No. 4,968,578, issued to Light et al, U.S. Pat. No. 5,037,718, issued to Light et at, and U.S. Pat. No. 5,284,731, issued to Tyagi et at. In each reference, dry toner is employed, typically having a particle size of less than 8 .mu.m (or less than 5 .mu.m) and a specially-coated image receiver (paper) is used.
In the thermally-assisted, direct transfer, the image receiver may be overcoated with a thermoplastic. The toner particles are embedded into the image receiver. An example of this process is disclosed in, e.g., above-mentioned U.S. Pat. No. 4,927,727.
Many variations of the foregoing thermally-assisted direct transfers are also known. For example, above-mentioned U.S. Pat. No. 5,037,718, U.S. Pat. No. 5,043,242, issued to Light et at, and U.S. Pat. No. 5,045,424, issued to Rimai et at, disclose selective polymer resins for the photoconductor surface and the image receiver without using release to achieve thermally-assisted transfer. Above-mentioned U.S. Pat. No. 5,284,731 employs heat plus an electric field, with no over-coated receiver. This is an electrostatically-assisted thermal transfer process that enables utilization of a reduced transfer temperature. The lower temperature and pressure requirements in turn lead to a reduction in "ghost" images.
The heat employed in these processes is sufficient to cause the toner particles to adhere at the point of contact, i.e., to fuse together, without melting, or flowing to form a single mass. The image receiver may be pre-heated, but it is important to avoid overheating, as this will result in reduced transfer efficiency.
Above-mentioned U.S. Pat. No. 5,037,718 is particularly relevant. This reference discloses a method for non-electrostatically transferring dry toner particles which comprise a toner binder and have a particle size of less than 8 .mu.m from the surface of an element which has a surface layer comprising a film-forming electrically insulating polyester or polycarbonate thermoplastic polymeric binder/resin matrix and a surface energy of not greater than about 47 dynes/cm, preferably about 40 to 45 dynes/cm, to a receiver which comprises a substrate having a coating of a thermoplastic condensation polymer on a surface of the substrate in which the glass transition temperature, T.sub.g, of the polymer is less than about 10.degree. C. above the T.sub.g of the toner binder and the surface energy of the thermoplastic polymer coating is about 38 to 43 dynes/cm by contacting the toner particles with the receiver which is heated to a temperature such that the temperature of the thermoplastic polymer coating on the receiver substrate during transfer is at least about 5.degree. C. above the T.sub.g of the thermoplastic binder, whereby virtually all of the toner particles are transferred from the surface of the element to the thermoplastic polymer coating on the receiver substrate and the thermoplastic coating is prevented from adhering to the element surface during transfer in the absence of a layer of a release agent on the thermoplastic polymer coating or the element. After transfer, the receiver is separated from the element while the temperature of the thermoplastic polymer coating is maintained above the T.sub.g of the thermoplastic polymer. The method is said to be provide images having high resolution and low granularity from very small size toner particles. However, this method must use a specially coated receiver to enhance the transfer efficiency, so it limits the use of the method for making images on plain paper.
Liquid toner tends to comprise particles even smaller than those of the dry toner disclosed in the above-discussed references, typically on the order of 1 .mu.m and less. It would seem at first blush that the methods used for depositing dry toner having relatively small toner size (&lt;8 .mu.m) could be used with liquid toner.
However, the office environment requires non-toxic, non-hazardous materials. The conventional liquid toning process provides the hard copies having carded out liquids which are no longer acceptable in the office environment. For example, in the so-called Benny Landa process, with reference to the E-1000 electrophotographic printer, liquid toner is used, having a small particle size on the order of less than 1 .mu.m. The transfer of toner is performed electrostatically, using the liquid toner, which puts liquid onto the paper, requiring drying after the transfer. The drying operation results in vapor emitted into the atmosphere, making this process unsuitable for office environments, due to toxicological concerns.
Thus, it is necessary to dry out the liquid from the toner imaging before the toner can be tranferred into the final image receiver (paper, plastic film, and the like). However, there are many problems involved, which make the transfer of a dried liquid toner image into another image receiver, or print medium, more difficult due to the following reasons:
(a) the adhesive force between the toner and the photoconductor surface becomes stronger when the particle size becomes smaller, especially at sizes below 1 .mu.m; PA1 (b) furthermore, that adhesive force even becomes stronger when the liquid carrier is eliminated or minimized; PA1 (c) the liquid toner particles tend to lose the charge when the liquid carrier is removed; and PA1 (d) some image receivers, such as plain paper, exhibit a very rough surface, and reduce the contact between the image carrying toner and the paper surface, thereby reducing the image transfer efficiency. PA1 (a) the photoconductor layer having the release layer thereon; PA1 (b) a roller spaced from the photoconductor layer; PA1 (c) means for directing the image receiver between the photoconductor layer and the roller such that the image receiver contacts both the release layer and the roller; PA1 (d) at least one conformable layer, supporting the photoconductor layer and the image receiver thereon; and PA1 (e) a source of heat and pressure against the image receiver for transferring the image to the image receiver.
Thus, there remains a need for a direct, one-step transfer of liquid toner from the photoconductor drum to an image receiver that can comprise materials other than specially coated papers. The direct transfer must be performed in a manner that will be acceptable in office environments and that overcomes the adhesive force problems listed above.